METHOD FOR OPERATING A BRAKING SYSTEM OF A MOTOR VEHICLE

Description

[0001] The present invention relates to a method for operating a braking system of a motor vehicle.

[0002] US 2010 / 0276240 A1 discloses a method for controlling a braking system for a vehicle. In this method, a master brake cylinder is separated from a wheel brake cylinder by means of a separation valve. The master brake cylinder is configured to detect an actuation of a brake input element. The flow of a brake fluid from a reservoir to a wheel brake cylinder is controlled by means of a control valve, thereby adjusting a braking pressure at an associated wheel.

[0003] WO 2019 / 197084 A1 discloses a braking system for a two-axle vehicle. Furthermore, WO 2018 / 177785 A1 discloses a method for compensating for low actuator dynamics of a mechanical brake of a motor vehicle. Furthermore, DE 10 2020 204 345 A1 discloses a method for operating a vehicle braking system. US 2020 / 0079219 A1 discloses a distribution of braking force between a friction brake and a regenerative brake in a hybrid vehicle. DE 10 2016 217 680 A1 discloses a method for operating a vehicle with a traction battery and a braking system. DE 10 2013 224 143 A1 discloses a method for operating a braking system of a vehicle.

[0004] The object of the present invention is to provide a solution which enables particularly good cleaning of respective wheel brakes of a motor vehicle with a particularly high energy recuperation of braking energy, wherein a brake pedal feel of a driver is particularly little influenced.

[0005] This object is achieved by the subject matter of the independent claim. Further possible embodiments of the invention are disclosed in the subclaims, the description, and the figures.

[0006] The invention relates to a method for operating a braking system of a motor vehicle. This braking system of the motor vehicle comprises a friction braking device and a regenerative braking device. The friction braking device is designed to convert the kinetic energy of the motor vehicle into thermal energy via friction. The friction braking device comprises a wheel brake designed as a respective friction brake for each wheel of the motor vehicle. The respective wheel brakes can be designed, for example, as disc brakes or as drum brakes. The regenerative braking device is designed to convert the kinetic energy of the motor vehicle into electrical energy. For this purpose, the regenerative braking device can have a generator. Particularly in vehicles with regenerative braking, respective friction brakes are used to a significantly lesser extent for deceleration than in vehicles that are decelerated exclusively via friction brakes.This can quickly and frequently lead to suboptimal friction coefficients at the wheel brakes, particularly due to corrosion, which can negatively impact braking performance, including the vehicle's acoustics. This may require the friction brakes to be replaced particularly frequently. Reducing the regenerative braking component in favor of increasing the hydraulic braking component will result in increased vehicle fuel consumption.

[0007] To overcome these disadvantages, the method provides for determining the need for cleaning of each wheel brake of the friction brake system of the motor vehicle. This need for cleaning can arise, for example, due to corrosion of the respective wheel brakes. This means that the respective wheel brakes must be cleaned if they show a certain degree of corrosion. The method also provides for determining, based on a brake pedal position of a brake pedal mechanically decoupled from the friction brake system, a deceleration of the motor vehicle desired by the driver and a braking force required for the desired deceleration of the motor vehicle. The brake pedal position can be determined, for example, using a sensor device. The driver can therefore specify how hard the motor vehicle should be braked by adjusting the brake pedal.Mechanical decoupling of the brake pedal from the friction brake system means that the brake pedal is not directly hydraulically coupled to the respective wheel brakes. This means that the brake pedal does not directly displace brake fluid in a hydraulic brake circuit connected to the wheel brake. Instead, a control signal is generated depending on the position of the brake pedal, which specifies the hydraulic pressure to be applied to the wheel brakes. This hydraulic pressure is adjusted by an actuator upon receipt of the control signal.

[0008] A first, predetermined portion of the braking force required for the desired deceleration of the motor vehicle is distributed between the regenerative braking system, and the remainder of this required braking force is distributed between the friction braking system. The motor vehicle is thus decelerated by both the regenerative braking system and the friction braking system. The braking force provided by the friction braking system is distributed between the wheel brakes depending on the determined cleaning requirements of the respective wheel brakes. The friction braking system can, for example, comprise respective hydraulic brakes, with each wheel of the motor vehicle being assigned a hydraulic brake. To distribute the braking force between the wheel brakes, a respective braking pressure at the wheel brakes can be set using a hydraulic fluid.The greater the respective cleaning requirement of a respective wheel brake, the greater the proportion of the remainder of the required braking force is applied to this wheel brake in order to clean this wheel brake by applying the braking force, in particular to free it from signs of corrosion. In particular, the braking system can be operated in a cleaning mode in which the cleaning requirement of the respective wheel brakes is determined and the remainder of the required braking force is distributed among the wheel brakes of the friction brake device depending on the determined cleaning requirement. The method thus enables a particularly large proportion of braking energy to be recovered via the recuperation braking device, thereby enabling the motor vehicle to be operated particularly efficiently and with particularly low energy consumption. On the other hand, targeted cleaning of respective wheel brakes of the braking system is enabled by dividing the braking force.A disadvantage in pedal feel, particularly due to the different distribution of braking force to the respective wheel brakes depending on the respective cleaning requirements of the wheel brakes, can be avoided by mechanically decoupling the brake pedal from the friction brake system. This can result in particularly high driving comfort for the vehicle's occupants, regardless of the respective distribution of braking force between the regenerative braking system and the friction brake system, or between the respective wheel brakes of the friction brake system.

[0009] The invention provides that the braking force distributed among the wheel brakes of the friction brake device is distributed among the wheel brakes depending on at least one predetermined stability criterion that must be observed. In this case, the predetermined stability criterion can predetermine respective ranges for ratios of braking force components applied to the respective wheel brakes in comparison between the respective wheel brakes. This means that the stability criterion predetermines how much the braking force components applied to different wheel brakes may differ from one another. The maximum differences can be predetermined in absolute or relative terms. By adhering to the predetermined stability criterion, it can be ensured that the vehicle does not run away during deceleration.This ensures that the stability of the vehicle is not negatively affected during deceleration and is particularly high.

[0010] Alternatively or additionally, the invention provides that when an anti-lock braking system and / or an electronic stability control system intervenes as driving dynamics control systems in the driving dynamics of the motor vehicle, the distribution of the braking force between the wheel brakes is specified by the driving dynamics control system. This means that the respective driving dynamics control system has a higher priority than the distribution of the required braking force based on the cleaning requirement. As soon as the respective driving dynamics control system intervenes in the braking of the motor vehicle for stabilization, the distribution of the braking force between the wheel brakes is specified by the driving dynamics control system. In this case, the driving dynamics system can specify absolute values ​​for the respective braking force components and / or limit values ​​or key data for the distribution of the braking force.The anti-lock braking system (ABS) counteracts possible wheel locking when a vehicle is braking by reducing the brake pressure. This allows the vehicle to maintain steerability and directional stability during braking. ABS prevents the vehicle's wheels from completely locking. This can be used to set a slip rate that specifies the extent to which individual wheel revolutions deviate from the total distance traveled. This means that during braking, the ABS specifies how far the wheels roll and the distance to which the wheels completely lock during braking. In addition, the ABS can be set to distribute the braking force between the front and rear axle wheels.Electronic Stability Control (ESC) is an electronically controlled driver assistance system for motor vehicles that counteracts skidding by selectively braking individual wheels using the corresponding wheel brakes. ESC counteracts skidding of the vehicle at the limit of cornering, both in the case of oversteer and understeer, by selectively braking individual wheels, thus ensuring the driver maintains control of the vehicle. Oversteer is corrected by braking the outside front wheel, and understeer by braking the inside rear wheel. Thus, ESC specifies the appropriate braking force for the wheel brakes to stabilize the vehicle.If, for example, the ESC detects a skidding of the vehicle, the braking force distribution determined based on the cleaning requirements of the wheel brakes is discarded and the braking force is instead distributed between the wheel brakes by the ESC. To do this, the ESC can specify a distribution of the braking force between the wheel brakes. In this case, it is possible for the ESC to distribute the required braking force based on the brake pedal position, or for the ESC to specify the braking force to be implemented, which deviates from the required braking force determined based on the brake pedal position. By specifying the braking force distribution through the activated driving dynamics control system, a particularly high level of stability of the vehicle's driving dynamics can be achieved.

[0011] Alternatively or additionally, the invention provides that a maximum possible recuperation power of the regenerative braking system is selected as the first predetermined portion of the braking force. This allows a particularly large amount of energy to be recovered through recuperation during deceleration of the motor vehicle, allowing the motor vehicle to be operated particularly efficiently and with low consumption. Thus, the recuperation capacity of the regenerative braking system is fully utilized to access the maximum possible recuperation power of the regenerative braking system.

[0012] In a possible further development of the method, it is provided that, depending on the braking force assigned to the respective wheel brake, a hydraulic braking pressure to be applied to the respective wheel brake for adjusting the braking force is determined via a stored allocation rule, and this determined braking pressure is applied to the assigned wheel brake. As an allocation rule, a characteristic map can be stored for each wheel brake, which map specifies a relationship between the respective hydraulic braking pressure applied to the wheel brake and the braking force resulting at the respective wheel brake in the form of a braking torque. Consequently, it is determined which braking force proportion of the braking force required for the desired deceleration is to be applied to the respective wheel brake based on the distribution.The allocation rule determines which hydraulic brake pressure must be applied to this wheel brake in order to ensure that the determined braking force component acts on this wheel brake. This determined hydraulic brake pressure is then applied to the assigned wheel brake. To distribute the remainder of the required braking force intended for the friction brake system among the wheel brakes, respective wheel brakes are selected based on their respective cleaning requirements, and their respective braking force component is assigned to the selected wheel brakes. The sum of the braking force components of all selected wheel brakes, including the first portion of the required braking force assigned to the regenerative braking system, corresponds to the total braking force required for the desired deceleration of the vehicle.Using the assignment rule, it can be determined with particular precision which hydraulic brake pressure must be applied to each wheel brake in order to ensure that the braking force component specified for that wheel brake is applied to that wheel brake. This hydraulic brake pressure can be adjusted using an actuator on the respective wheel brake. This makes it possible to mechanically decouple the brake pedal from the setting of the hydraulic brake pressure on the respective wheel brakes. As a result, for a fixed required braking force, the brake pedal position and brake pedal resistance are independent of the distribution of the braking force between the friction braking system and the regenerative braking system, as well as independent of the distribution of the braking force between the respective wheel brakes. The driver cannot therefore feel any differences on the brake pedal for each different distribution of the required braking force.Consequently, there is no disadvantage in pedal feel.

[0013] In this context, a possible development of the method can provide for the at least one stability criterion to be met if the respective braking force assigned to the wheel brakes corresponds at most to a maximum braking force for the respective wheel brake and / or a braking force ratio between respective wheel brakes lies within a permitted range for this braking force ratio. In other words, application parameters can be specified which specify limit values ​​or key data for the braking force distribution to the wheel brakes. An individual maximum braking force can be specified for each wheel brake, or a maximum braking force can be specified which must be observed for each of the wheel brakes. The permitted range for the braking force ratio between the respective wheel brakes specifies how much the respective braking force components of the individual wheel brakes may differ from one another.Excessive differences between the braking force components applied to the wheel brakes can negatively impact the vehicle's driving dynamics. This impact on driving dynamics can cause the vehicle to skid or deviate from its lane. Adhering to the stability criterion enables particularly safe operation of the vehicle.

[0014] In a further possible embodiment of the invention, it is provided that when the anti-lock braking system and / or the electronic stability control as driving dynamics control systems intervene in the driving dynamics of the motor vehicle, the distribution of the braking force between the recuperation braking device and the friction braking device is specified by the driving dynamics control system. This means that the respective driving dynamics control system has a higher priority than the distribution of the required braking force carried out depending on the cleaning requirement. As soon as the respective driving dynamics control system intervenes in the braking of the motor vehicle for the purpose of stabilization, the distribution of the braking force between the recuperation braking device and the friction braking device is specified by the driving dynamics control system. In this case, the driving dynamics system can specify absolute values ​​for the respective braking force components and / or limit values ​​orSpecify key data for the distribution of braking force. If, for example, the ESC detects a skidding of the vehicle, the braking force distribution determined based on the cleaning requirements of the wheel brakes is discarded and the braking force is distributed between the wheel brakes by the ESC instead. To do this, the ESC can specify a distribution of braking force between the regenerative braking system and the friction braking system. In this case, it is possible for the ESC to divide the required braking force based on the brake pedal position, or for the ESC to specify the braking force to be implemented, which deviates from the required braking force determined based on the brake pedal position. By specifying the braking force distribution through the activated driving dynamics control system, a particularly high level of stability of the vehicle's driving dynamics can be achieved.

[0015] In a further development of the invention, the braking force distributed among the wheel brakes is distributed evenly among all wheel brakes when it is determined that no cleaning is required. In other words, a respective braking force component is provided for each of the wheel brakes, which is the same for all wheel brakes. This allows for particularly uniform deceleration of the motor vehicle and minimizes the risk of the vehicle skidding during braking.

[0016] In this context, it can be provided in a possible further development of the invention that, if the braking force required for the predetermined deceleration of the motor vehicle is less than or equal to the maximum possible recuperation power of the recuperation braking device and the cleaning requirement has been determined, a lower recuperation power than the maximum possible recuperation power of the recuperation braking device is selected as the first predetermined part of the braking force and the remainder of the required braking force is distributed between the friction braking device.If the braking force required for the specified deceleration of the motor vehicle is less than or equal to the maximum possible recuperation power of the recuperation braking system, and if the maximum possible recuperation power of the recuperation braking system were selected as the first specified portion of the braking force, then the braking force available for the friction braking system would be 0. Thus, cleaning of the wheel brakes of the friction braking system would not be possible. For this reason, once the cleaning requirement has been determined, the recuperation power lower than the maximum possible recuperation power of the recuperation braking system is selected as the first specified portion of the braking force, provided that the braking force required for the specified deceleration of the motor vehicle is less than or equal to the maximum possible recuperation power.If the braking force required for the specified deceleration of the motor vehicle is greater than the maximum possible recuperation power of the recuperation braking system, then the maximum possible recuperation power of the recuperation braking system can be selected as the first specified portion of the braking force. In other words, a distinction is made between cases. If it is determined that recuperation deceleration alone is sufficient for the specified deceleration of the motor vehicle, but at least one of the wheel brakes is dirty and therefore requires cleaning, then less deceleration is achieved via the recuperation braking system than would be possible with the maximum possible recuperation power of the recuperation braking system, with the motor vehicle additionally decelerated via hydraulic braking using the friction brake system.In a second case, the recuperation deceleration is not sufficient to decelerate the motor vehicle by the specified deceleration, whereby the remainder of the required braking force is provided by the friction brake device, with this braking force being distributed between the wheel brakes depending on the respective cleaning requirements of the respective wheel brakes. One recuperation objective of the method is to be able to recover a particularly large proportion of braking force via recuperation power. To achieve this recuperation objective, the braking force is divided between the friction brake device and the recuperation braking device, whereby in addition to cleaning at least one wheel brake, as much braking force as possible is to be recovered via the recuperation braking device. This enables the motor vehicle to be operated in a particularly energy-efficient manner.

[0017] In a further possible embodiment of the invention, it is provided that a larger proportion of the braking force of the friction brake device is distributed between the wheel brakes of all rear wheels of the motor vehicle compared to the wheel brakes of all front wheels. If the motor vehicle is a motorcycle, then it is possible for the larger proportion of the braking force of the friction brake device to be distributed between the wheel brakes of the rear wheel of the motorcycle compared to the wheel brakes of the front wheel of the motorcycle. However, if the motor vehicle is a car, then it is provided that the larger proportion of the braking force of the friction brake device is distributed between the wheel brakes of both rear wheels of the motor vehicle compared to the wheel brakes of the front wheels.Disc brakes on the rear axle of a motor vehicle, in particular, require particularly intensive cleaning, as they experience lower energy input than front-wheel brakes during normal customer operation. By distributing the larger proportion of the braking force of the friction brake system to the wheel brakes of all rear wheels of the motor vehicle compared to the wheel brakes of the front wheels, reliable cleaning of the wheel brakes of each rear wheel of the motor vehicle can be achieved.

[0018] In a further possible embodiment of the invention, the cleaning requirement of the respective wheel brakes is determined using a model as a function of the brake pressure applied to the respective wheel brakes and the braking frequency, in particular the brake pressure applied to the respective wheel brakes and the braking frequency over their service life. The model can therefore be a type of energy calculator which can calculate, over the service life of the respective wheel brakes, when and at which wheel brake, and for how long, braking was carried out due to a respective braking force input from a respective wheel brake, and how corrosion develops on the respective wheel brakes accordingly. The model can therefore be used to particularly accurately estimate how the respective cleaning requirement of the respective wheel brakes develops over their service life.This advantageously eliminates the need for additional sensors to determine the cleaning requirements of each wheel brake. The model can store each braking event at the wheel brakes with precise resolution regarding duration and / or brake pressure, allowing the cleaning requirements for each wheel brake to be determined with particularly high accuracy.

[0019] In a further possible embodiment of the invention, the braking force to be applied to a respective wheel brake by means of the friction braking device is set to be higher, the greater the respective determined cleaning requirement of this wheel brake. The braking force applied to the respective wheel brake can grind down this wheel brake, with the grinding effect being greater the higher the braking force applied to this wheel brake. This means that for particularly effective grinding down of, for example, signs of corrosion, a particularly high braking force is applied to the wheel brake to be cleaned. A particularly good cleaning effect can thus be achieved by applying the braking force to the respective wheel brake.

[0020] Further features of the invention can be derived from the following description of the figures and from the drawings. The features and combinations of features mentioned above in the description, as well as the features and combinations of features shown below in the description of the figures and / or in the figures alone, can be used not only in the respective combinations specified, but also in other combinations or on their own, without departing from the scope of the invention.

[0021] The drawing shows in the single figure a process diagram for a method for operating a braking system of a motor vehicle.

[0022] The drawing shows Fig. 1A process diagram for a method for operating a braking system of a motor vehicle, wherein the braking system comprises a friction braking device and a regenerative braking device. The friction braking device is configured to decelerate the motor vehicle via respective friction brakes, which are assigned to respective wheels of the motor vehicle and are referred to below as wheel brakes. The regenerative braking device is configured to decelerate the motor vehicle, wherein kinetic energy of the motor vehicle during deceleration is converted into electrical energy via the regenerative braking device, which in turn can be stored in a battery of the motor vehicle. As a result, the motor vehicle can be driven in a particularly energy-efficient manner using the energy stored in the battery of the motor vehicle by means of the regenerative braking device.

[0023] In the method, a first method step V1 is provided for determining the cleaning requirement of each wheel brake of the friction brake system of the motor vehicle. The cleaning requirement of each wheel brake can be determined, in particular, using a model as a function of the brake pressure applied to the respective wheel brakes and the braking frequency during each braking event, in particular over the service life of the wheel brakes.

[0024] In a second method step V2 of the method, it is provided that a braking force required for a deceleration of the motor vehicle desired by a driver is determined. For this purpose, a brake pedal position of a brake pedal mechanically decoupled from the friction brake device is analyzed and the desired deceleration is determined as a function of the determined brake pedal position. The driver can thus specify how much the motor vehicle should be decelerated by pressing the brake pedal in different brake pedal positions. The method thus determines how much the motor vehicle is to be decelerated according to the driver's wishes and what braking force is necessary to bring about this deceleration of the motor vehicle. By decoupling the brake pedal from the friction brake device, it is avoided that the driver can sense a distribution of the braking force between the respective wheel brakes of the friction brake device via a resistance of the brake pedal.Because the brake pedal is decoupled from the friction brake system, the brake pedal resistance for the driver is the same for each braking force distribution at a desired deceleration determined by the brake pedal position. Thus, there is no so-called pedal feel disadvantage.

[0025] In a third method step V3 of the method, a first, predetermined portion of the braking force required for the desired deceleration of the motor vehicle is distributed to the regenerative braking system, and the remainder of the required braking force is distributed to the friction braking system. The portion of the braking force provided by the friction braking system is distributed to the wheel brakes depending on the determined cleaning requirement of the respective wheel brakes.

[0026] In particular, a maximum possible recuperation power of the recuperation braking system is selected as the first predetermined portion of the braking force in order to be able to recover a particularly large amount of braking energy and thereby operate the motor vehicle particularly energy-efficiently. If the braking force required for the predetermined deceleration of the motor vehicle is less than or equal to the maximum possible recuperation power of the recuperation braking system and, furthermore, the cleaning requirement for at least one wheel brake of the friction braking system has been determined, then a lower recuperation power than the maximum possible recuperation power of the recuperation braking system is selected as the first predetermined portion of the braking force, and the remainder of the required braking force is distributed among the respective wheel brakes of the friction braking system.The greater the determined cleaning requirement of a particular wheel brake, the more braking force can be applied to that wheel brake to grind it free of any contamination. To avoid jeopardizing the stability of the vehicle during braking, the remaining braking force to be distributed among the wheel brakes can be distributed among the wheel brakes depending on a predefined stability criterion. It can be specified that the stability criterion is met if the respective braking force assigned to the wheel brakes corresponds at most to a maximum braking force for the respective wheel brake and / or a braking force ratio between the respective wheel brakes lies within an approved range for this braking force ratio.Furthermore, it can be provided that when an anti-lock braking system and / or an electronic stability control system as driving dynamics control systems intervene in the driving dynamics of the motor vehicle, the distribution of the braking force between the recuperation braking device and the friction braking device and / or between the wheel brakes is specified by the driving dynamics control system.

[0027] A larger proportion of the braking force of the friction brake system can be distributed to the wheel brakes of all rear wheels of the vehicle compared to the wheel brakes of the respective front wheels. If it is determined that no cleaning of the individual wheel brakes is required, the braking force to be distributed among the wheel brakes can be distributed evenly among all wheel brakes.

[0028] In the method, during manual braking, in which the driver of the motor vehicle specifies how strongly the motor vehicle is to be decelerated via the respective brake pedal position, a hydraulic component of the braking torque, in particular the braking force required for the desired deceleration of the motor vehicle, is distributed in a targeted manner and within the stability limits of the motor vehicle to axles or specifically to individual wheels, in particular the wheel brakes of the motor vehicle assigned to the wheels. The aim of this is to increase the energy input to the grinding wheel brakes and thus the grinding effect. This makes it possible to apply even a small hydraulic braking torque component, in particular during braking with a recuperative component, to a targeted manner to an axle or to defined wheels of the motor vehicle, regardless of the design brake force distribution.Negative influences on pedal feel can be avoided by using a decoupled braking system, as the brake pedal is mechanically decoupled from the friction brake device. This decoupled braking system can be a simulator system that includes a travel simulator and can be designed, for example, as a 1-box system. The term 1-box system describes the design of the braking system, with all components of the braking system being housed in a common housing and therefore in a box. The brake pedal can be directly connected to this box. An electromechanical brake booster, an electronic stability control system and a brake fluid reservoir can be arranged in the common housing. The simulator system describes one possible form of mechanical decoupling of the brake pedal from the friction brake device.Brake fluid displaced by the brake pedal can be diverted to the brake fluid reservoir, which can also be referred to as a simulator. This prevents brake fluid from being directly displaced to the wheel brakes by the brake pedal. Instead, the brake pressure in the respective wheel brakes is built up by a separate motor via the respective hydraulic lines.

[0029] In a coupled braking system with a vacuum brake booster or an electromechanical brake booster, the brake pedal feel changes significantly due to the distribution of braking force to selected wheels. Brake pedal feel is determined by the relationship between brake pedal travel or brake pedal force and the deceleration of the vehicle. By hydraulically switching off individual wheels during braking with the coupled braking system, the driver needs to move less volume of brake fluid to generate a specified pressure in the remaining wheel brakes, due to a steeper travel-pressure characteristic curve in this case. In this case, however, the specified deceleration is not achieved because the braking torque generated at any given time acts on fewer wheels overall.

[0030] In the decoupled braking system, which can particularly be a brake-by-wire braking system, a travel-to-force curve is predetermined by mechanical design when the brake pedal is actuated, independent of hydraulic conditions in the friction brake system. The deceleration to the respective brake pedal position is triggered by a pressure actuator in the friction brake system that is decoupled from the brake pedal and can be specified by software functions in the braking system.

[0031] The method enables the brake pedal feel and thus the brake pedal travel deceleration behavior to be adjusted so that it corresponds to the normal deceleration behavior expected by the driver, i.e., deceleration behavior without active brake torque distribution to individual wheels or braking across all wheels of the vehicle. For this purpose, characteristic curves and / or characteristic maps can be stored in a control unit of the braking system for each wheel brake, which map the braking torque that occurs at the wheel for a set brake pressure. Using a function in the control unit, the brake pedal position, which represents a driver command, can be read in and a desired total wheel braking torque can be determined. This desired total wheel braking torque results from an applied standard brake pedal travel deceleration behavior for braking the vehicle across all wheels, i.e., without functional intervention.If individual wheels or axles are hydraulically isolated during this braking process, the function uses the stored characteristic curves to determine the appropriate brake pressure for each wheel brake, which results in the desired total wheel braking torque at the active wheel brakes. This applied brake pressure is set at the respective wheel brakes using a pressure regulator of the friction brake system. This means that the brake pedal travel-deceleration behavior is identical to the standard brake pedal travel-deceleration behavior, and the active function is either not noticeable or barely noticeable to the driver.

[0032] In this process, the brake pressure in the friction brake system is measured or calculated. Application parameters can be specified for the function, which define which wheel brakes and / or axles may be disconnected and / or up to what deceleration the respective wheel brakes and / or axles may be disconnected. The function described is intended to have the least impact on vehicle stability and safety. For this reason, vehicle stability functions and vehicle safety functions have a higher priority. If the function is to be terminated and braking is to be resumed via all wheel brakes, the corresponding wheel brakes are released via corresponding valves and the brake pressure is adjusted via the pressure controller according to the desired total wheel braking torque on all wheels of the vehicle. The function can be activated as needed and thus dependent on an estimated condition of the wheel brakes.

[0033] Due to the targeted distribution of the hydraulic braking torque component, particularly during braking operations with a recuperative component, to individual axles or wheels, in particular the wheel brakes assigned to these axles or wheels, a fuel consumption disadvantage can be avoided. Furthermore, the use of this function can avoid a pedal feel disadvantage during manual braking operations. An algorithm can be used to determine and distribute the hydraulic braking torque. This algorithm is used to adapt the deceleration of the vehicle depending on the driver's braking command and the active wheel brakes in question, so that the brake pedal feel for the driver remains the same for a given driver braking command, regardless of the active wheel brakes.

[0034] In a brake control system control unit, and thus the control device of the motor vehicle's braking system, a driver's braking request can be divided into a generator and a hydraulic component, depending on the level of available hydraulic generator torque. The hydraulic component can then be distributed to the wheel brakes assigned to the axles or wheels that potentially have the greatest cleaning requirement and thus the greatest need for grinding. In this method, a desired total wheel braking torque equivalent to braking across all wheels is determined based on the driver's braking request, and a brake pressure corresponding to braking across the selected wheels or axles is calculated based on stored characteristic curves. This brake pressure is set via the pressure control unit in the friction brake system.In particular, the distribution of the remaining required braking force to the wheel brakes of the friction brake system is imperceptible to the driver of the motor vehicle, with the stability of the vehicle being affected very little, or even not at all. The cleaning requirements of the respective wheel brakes can be determined using an algorithm that can add up the energy input for each wheel brake. Generally, disc brakes on a rear axle require a greater cleaning of contaminants, particularly signs of corrosion, since they experience a lower energy input than front axle brakes during normal customer operation.

[0035] Overall, the invention shows how a function for improving the friction properties of respective wheel brakes in vehicles with decoupled braking systems can be achieved. List of reference symbols

[0036] V1 to V3 respective process steps

Claims

1. Method for operating a braking system of a motor vehicle, having a friction brake device and a regenerative brake device, in which method a cleaning requirement of respective wheel brakes of the friction brake device of the motor vehicle is determined (V1), and, on the basis of a brake pedal position of a brake pedal mechanically decoupled from the friction brake device, a deceleration of the motor vehicle desired by the driver is determined and a braking force required for the desired deceleration of the motor vehicle is determined (V2), wherein a first, predetermined part of this braking force is distributed to the regenerative brake device and the remainder of the required braking force is distributed to the wheel brakes of the friction brake device (V3), wherein the braking force to be provided by the friction brake device is distributed to the wheel brakes depending on the determined cleaning requirement of the respective wheel brakes, wherein - the braking force distributed to the wheel brakes of the friction brake device is distributed to the wheel brakes depending on at least one predetermined stability criterion to be met, and / or - when an anti-lock braking system and / or an electronic stability control system as driving dynamics control systems intervene in the driving dynamics of the motor vehicle, the distribution of the braking force between the wheel brakes is predetermined by the driving dynamics control system, and / or - as the first predetermined part of the braking force, a maximum possible regenerative power of the regenerative brake device is selected.

2. Method according to claim 1, wherein, depending on the braking force assigned to the particular wheel brake, a hydraulic braking pressure to be applied to the particular wheel brake for adjusting the braking force is determined via a stored allocation rule and this determined braking pressure is applied to the assigned wheel brake.

3. Method according to either of the preceding claims, wherein the at least one stability criterion is met if the particular braking force assigned to the wheel brakes corresponds at most to a maximum braking force for the particular wheel brake and / or a braking force ratio between respective wheel brakes is within an approved range for this braking force ratio.

4. Method according to any of the preceding claims, wherein when the anti-lock braking system and / or the electronic stability control as driving dynamics control systems intervene in a driving dynamics of the motor vehicle, the distribution of the braking force between the regenerative brake device and the friction brake device is predetermined by the driving dynamics control system.

5. Method according to any of the preceding claims, wherein the braking force distributed to the wheel brakes is distributed evenly to all wheel brakes when it is determined that there is no cleaning requirement.

6. Method according to any of the preceding claims, wherein if the braking force required for the predetermined deceleration of the motor vehicle is less than or equal to the maximum possible regenerative power of the regenerative brake device and the cleaning requirement has been determined, a lower regenerative power than the maximum possible regenerative power of the regenerative brake device is selected as the first predetermined part of the braking force and the remainder of the required braking force is distributed to the friction brake device.

7. Method according to any of the preceding claims, wherein a larger proportion of the braking force of the friction brake device is distributed to wheel brakes of all rear wheels of the motor vehicle compared with wheel brakes of all front wheels.

8. Method according to any of the preceding claims, wherein the cleaning requirement of the respective wheel brakes is determined via a model depending on the brake pressure applied to the respective wheel brakes and the braking frequency, in particular depending on the brake pressure applied to the respective wheel brakes over their service life and the braking frequency.

9. Method according to any of the preceding claims, wherein the higher the particular determined cleaning requirement at a particular wheel brake, the higher the braking force to be applied to this wheel brake by means of the friction brake device is set.